In electrostatographic imaging processes, a charge pattern latent image is formed on a photoconductive surface and then is developed on that surface, or on another surface to which the charge pattern is transferred, by contact with finely divided, oppositely charged toner particles. The toner can be applied by means of a liquid developer, which is a colloidal suspension of polymeric toner particles in a volatile carrier liquid or by means of a dry developer which is a physical mixture of relatively large carrier particles (magnetic carrier particles being used in the well-known magnetic brush method of development) and of smaller polymeric toner particles.
The polymer of which the toner particles are made must meet exacting requirements in order to form images of high quality. For instance, the toner particles must be very small in order to develop the image with a high degree of resolution of fine details. The toner polymer must fuse at a reasonably low temperature in order to facilitate the fixing of the toner image to paper or other receiver sheets by means of heated fusing rolls. The polymer must also have a high degree of melt cohesive strength in order to avoid the image defect known as "hot offset." This occurs in electrophotographic copying machines which use a heated fusing roll to fix the toner image to a paper sheet or other receiver sheet after transfer from the photoconductive surface. Unless the toner polymer has sufficient melt cohesive strength, some of it will stick to the fusing roll during the fusing step and will transfer to a subsequent sheet of paper, thus forming on it an "offset" or "ghost" image in addition to the desired one.
In order to grind a toner polymer to a small particle size as required for development of images with a high degree of resolution, the polymer must be brittle. In other words, it must have a high degree of grindability. Many polymers which would otherwise be useful as toners cannot be ground easily to very small sizes and hence are not suitable for making high-resolution toners.
Still further, toners to be used in color imaging processes, which use the subtractive principle of color reproduction with combinations of toners of three or four different colors, must be transparent to all but certain wavelengths of light. This rules out a number of otherwise satisfactory polymers which are hazy or are otherwise inadequately transparent.
A need has existed, therefore, for an improved toner composition, made from a polymer which can be ground easily to a very small particle size, which has sufficient cohesivity to resist hot offset in the fusing step, which can be fixed to the receiver by fusion at low temperature, and which is transparent to selected wavelengths of light when fused on the receiver sheet.